1. Field of the Invention
The present invention relates to a communication apparatus for performing broadband transmission by using tens to hundreds of subcarriers or more, particularly to an array antenna transceiver for performing adaptive antenna control and more particularly relates to calibration of the phase/amplitude characteristic between antennas in an array antenna transceiver.
2. Description of the Prior Art
For a cellular mobile communication system, a method is studied which forms a transmission directivity pattern for improving a signal in speed and quality, increasing a subscriber capacity, and increasing a transmission gain in the incoming direction of a desired signal and decreasing the interference to other users by using an array antenna transceiver constituted by a plurality of antenna elements with a high correlation. In this case, the array antenna transceiver forms a transmission directivity pattern of a transmission signal toward the incoming direction obtained from a desired signal received by an array antenna.
In the case of an array antenna transceiver having a plurality of radio transmission sections, generally the amplitude and phase of a transmitting radio section connected to each antenna element independently fluctuate momently. Therefore, it is necessary to compensate the fluctuation of a phase and amplitude when forming a transmission directivity pattern. The above operation is referred to as calibration.
Conventionally, this type of the array antenna transceiver calibrating method purposes to compensate the phase (delay) and amplitude (gain) of each radio reception section which independently fluctuate momently by inputting a known calibration signal into each transmitting radio section connected to each antenna element and using a result of demodulating a calibration signal as described in a patent document 1 “An array antenna radio CDMA communication units” (disclosed date; Dec. 18, 1998).
FIG. 8 is a block diagram showing a conventional array antenna transceiver for calibration.
The conventional array antenna transceiver is constituted by an array antenna (801), a distributor 1 (803-1) to a distributor N (803-N), a transmitting radio section 1 (804-1) to a transmitting radio section N (804-N) to be connected to each antenna element, a transmission baseband processing section 1 (805-1) to a transmission baseband processing section N (805-N) for producing the calibration signal, an adder (807) for synthesizing distributor outputs, a receiving radio section (808), calibration signal demodulating section 1 (809-1) to a calibration signal demodulating section N (809-N), and a demodulation result processing section (810).
The array antenna (801) is constituted by N antenna elements (802-1 to 802-N). The N antenna elements (802-1 to 802-N) are closely arranged like an array, which makes it possible to form a desired transmission directivity pattern by controlling a phase/amplitude by each transmission base band processing section. In this case, the number of antenna elements N is set to 3 or more in order to differentiate the configuration of the array antenna (801) from a normal diversity configuration.
The transmission baseband processing section 1 (805-1) to transmission baseband processing section N (805-N) generate calibration signals and output them to the transmitting radio section 1 (804-1) to transmitting radio section N (804-N) to which the sections 805-1 to 805-N are connected. In this case, it is assumed that a communication signal and a calibration signal are multiplexed and code multiplexing is performed. Moreover, a communication signal to which phase/amplitude control is applied every user is corrected by using a calibration coefficient output from the demodulation result processing section (810).
The transmitting radio section 1 (804-1) to transmitting radio section N (804-N) receive outputs of the transmission baseband processing section 1 (805-1) to transmission baseband processing section N (805-N), apply digital/analog conversion, orthogonal modulation, frequency conversion, amplification, band limitation and so firth to the multiple signal of a communication signal and calibration signal for a user, and output the multiple signal to the distributor 1 (803-1) to distributor N (803-N).
The distributor 1 (803-1) to distributor N (803-N) receive multiple signals of user signals and calibration signals which are outputs of the transmitting radio section 1 (804-1) to transmitting radio section N (804-N) and fetch some of the multiple signals to output them to the adder (807).
The adder (807) receives outputs of the distributor 1 (803-1) to distributor N (803-N) and combines output signals in a radio band to output them to the receiving radio section (808).
The receiving radio section (808) applies band limitation, amplification, frequency conversion, orthogonal demodulation, and analog/digital conversion and so forth to the combined signal and outputs the signal to the calibration signal demodulating section 1 (809-1) to calibration signal demodulating section N (809-N).
The calibration signal demodulating section 1 (809-1) to calibration signal demodulating section N (809-N) extract calibration signals in input signals from the multiple signal which is output from the receiving radio section (808), detects a transmission route 1 demodulation symbol point (phase/amplitude information) to transmission route N demodulation symbol point (phase/amplitude information), and outputs the detected symbol points to the demodulation result processing section (810).
The demodulation result processing section (810) receives outputs of the calibration signal demodulating section 1 (809-1) to calibration signal demodulating section N (809-N), calculates the correction information on transmission routes, and outputs the information to the transmission baseband processing section 1 (805-1) to transmission baseband processing section N (805-N). In this case, when normalization is performed by the transmission baseband processing section 1 (805-1), it is not necessary to send an input from the demodulation result processing section (810) to the transmission baseband processing section 1 (805-1).
Then, operations of a conventional example are described below.
The transmission baseband processing section 1 (805-1) to transmission baseband processing section N (805-N) generate communication signals and calibration signals for users in a baseband and output the signals to the transmitting radio section 1 (804-1) to transmitting radio section N (804-N).
The transmitting radio section 1 (804-1) to transmitting radio section N (804-N) receive communication signals and calibration signals in a baseband for users, which are outputs of the transmission baseband processing section 1 (805-1) to transmission baseband processing section N (805-N), apply digital/analog conversion and frequency conversion from the baseband to a radio band to the signals, and output the signals to the distributor 1 (803-1) to distributor N (803-N).
The distributor 1 (803-1) to distributor N (803-N) receive communication signal with users, which are outputs of the transmitting radio section 1 (804-1) to transmitting radio section N (804-N) and calibration signals in the same band, distribute some power of the signals, and output the signals to the adder (807).
The adder (807) receives communication signals with users, which are outputs of the transmitting radio section 1 (804-1) to transmitting radio section N (804-N) and N multiplexed calibration signals, combines the signals in a radio band, and outputs them to the receiving radio section (808).
The receiving radio section (808) receives a multiple signal which is an output of the adder (807), applies band limitation, amplification, frequency conversion, orthogonal demodulation, and analog/digital conversion and so forth to the signal, and outputs the signal to the calibration signal demodulating section 1 (809-1) to calibration signal demodulating section N (809-N).
The calibration signal demodulating section 1 (809-1) to calibration signal demodulating section N (809-N) extract calibration signals in input signals from a multiple signal output from the receiving radio section (808), detect a demodulation symbol point 1 (phase/amplitude information) to a demodulation symbol point N (phase/amplitude information), and output the points to demodulation result processing section (810). It is possible to extract a calibration signal multiplexed on an input signal through a de-spreading operation.
The demodulation result processing section (810) receives outputs of the calibration signal demodulating section 1 (809-1) to calibration signal demodulating section N (809-N), calculates calibration coefficients serving as correction information on routes in accordance with the demodulation symbol point 1 (phase/amplitude information) added with the phase/amplitude fluctuation of a transmission route including the transmitting radio section (804-1) and the demodulation symbol point 2 (phase/amplitude information) to the demodulation symbol point N (phase/amplitude information) added with the phase/amplitude fluctuations of the transmission routes 2 to N including the transmitting radio section 2 (804-2) to transmitting radio section N (804-N) and output the calibration coefficients to the transmission a baseband processing section 1 (805-2) to transmission baseband processing section N (805-N).
Then, a phase/amplitude information extracting method is described below in detail. It is defined that a transmission route including the transmitting radio section 1 (804-1) always serves as a reference route to assume a symbol point obtained by demodulating a calibration signal output from the transmitting radio section 1 (804-1) by the calibration signal demodulating section 1 (809-1) as a reference symbol point S1 (FIG. 9). Moreover, when assuming a symbol point output from the transmitting radio section 2 (804-2) and demodulated by the calibration signal demodulating section 2 (809-2) as S2 (FIG. 9) and a symbol point output from the transmitting radio section N (804-N) and demodulated by the calibration signal demodulating section N (809-N) as Sn (FIG. 9), the demodulating result processing section (810) detects the phase difference θ2 between S1 and S2, amplitude ratio r2=B/A, and phase difference θn between S1 and Sn, and amplitude ratio rn=C/A. FIG. 10 shows the normalized reference symbol point S1. In this case, though values of the phase differences θ2 and θn and those of the amplitude ratios r2 and rn are not changed, r2=B/A=B′/1 and rn=C/A=C′/1 are effected. The demodulation result processing section (810) outputs values of θ2 and θn and those of r2 and rn to the transmission baseband processing section (805-1) to transmission baseband processing section (805-N).
The transmission baseband processing section 1 (805-1) to transmission baseband processing section N (805-N) correct communication signals which are phase/amplitude-controlled every user by using calibration coefficients output from the demodulation result processing section (810).
An array antenna transceiver having the above calibration means makes it possible to correct a phase/amplitude fluctuation by supplying calibration coefficients to the transmission baseband processing section 1 (805-1) to transmission baseband processing section N (805-N) even if a phase/amplitude fluctuation occurs in each transmission route while the transceiver is operated.
Therefore, the conventional array antenna transceiver shown in FIG. 8 makes it possible to form an accurate transmission directivity pattern by correcting a phase/amplitude fluctuation in each route for a communication signal with a user who forms a transmission directivity pattern by performing the weighting control according to phase/amplitude and then transmitting the communication signal.
[Patent Document 1]
Official gazette of Japanese Patent Laid-Open No. 10-336149
However, the above conventional calibrating method has the following problems.
Firstly, in the case of broadband transmission, components in a transceiver have different frequency characteristics (amplitude/phase). Therefore, to absorb the fluctuation of a frequency characteristic including an environmental change such as a temperature change or humidity change or aging, it is necessary to calibrate all subcarriers in all transmission routes in order to form an accurate transmission directivity pattern. Therefore, when applying a conventional calibrating method to a transceiver for performing broadband transmission through each antenna element by using hundreds to thousands of subcarriers, a problem occurs that it is necessary to calculate calibration coefficients of all subcarriers in all transmission routes in order to form an accurate transmission directivity pattern.
Secondly, a transmission baseband processing section for generating a calibration signal requires a calibration signal generating circuit for all subcarriers in all transmission routes and a calibration signal demodulating section requires a calibration signal demodulating section for all subcarriers in all transmission routes. Therefore, when applying a conventional calibrating method to a transceiver for performing broadband transmission through each antenna element by using hundreds to thousands of subcarriers, a problem occurs that the transceiver is greatly increased in size.
Thirdly, the number of routes to be calibrated is greatly increased and even if performing the time-sharing calibration every a plurality of subcarriers, the signal processing becomes complex. Moreover, to form an accurate transmission directivity pattern, it is necessary to calculate calibration coefficients by demodulating calibration signals simultaneously supplied to the largest possible number of transmission routes. Therefore, when applying the conventional calibrating method to a transceiver for performing broadband transmission through each antenna element by using hundreds to thousands of subcarriers, a problem occurs that the load of signal processing in calibration greatly increases.